This application is a national stage of PCT International Application No. PCT/EP2008/006640, filed Aug. 13, 2008, which claims priority under 35 U.S.C. §119 to German Patent Application No. 10 2007 040 439.7, filed Aug. 28, 2007, the entire disclosure of which is herein expressly incorporated by reference.
The present invention relates to an operating and diagnostic method for an SCR (selective catalytic reduction) catalytic converter with an SCR exhaust gas aftertreatment system having an ammonia storage capacity.
German patent document DE 102 54 843 A1 disclosed an operating and diagnostic method for an SCR catalytic converter with an SCR exhaust gas aftertreatment system having an ammonia storage capacity, where a signal of an exhaust gas sensor arranged downstream of the SCR catalytic converter in an exhaust gas line of the internal combustion engine is sensed with a sensitivity with regard to nitrogen oxide and ammonia. The amount of the reduction agent supplied to the SCR catalytic converter is changed for diagnosing the SCR exhaust gas aftertreatment system and an error is recognized, if the signal does not change as expected. While this method indeed permits a recognition of a malfunctioning of the SCR exhaust gas aftertreatment system in a reliable manner, it is relatively unspecific.
One object of the present invention, therefore, is to provide an operating and diagnosing method, which enables reliable operation and improved error treatment.
This and other objects and advantages are achieved by the operating and diagnostic method according to the invention, for an SCR exhaust gas aftertreatment system with an SCR catalytic converter with ammonia storage capacity, in which method an exhaust gas sensor with a sensitivity to nitrogen oxide and ammonia is used downstream of the SCR catalytic converter. In a normal operating mode, ammonia is supplied to the SCR catalytic converter corresponding to a filling level model for ammonia stored in the SCR catalytic converter, in such a way that the ammonia filling level of the SCR catalytic converter (5) is held at least approximately at a predefinable nominal value or in a predefinable nominal value range. By means of the exhaust gas sensor with sensitivity to nitrogen oxide and ammonia, the nitrogen oxide conversion capacity of the SCR exhaust gas aftertreatment can thereby be sensed on the one hand, while on the other hand, the ammonia filling level of the catalytic converter relevant for the nitrogen oxide can be controlled. In this manner, an optimum operation of the SCR exhaust gas aftertreatment system and an error diagnostics are enabled. An insufficient nitrogen oxide conversion or an underdosing of urea is noted by means of an increased nitrogen oxide concentration and possibly corrected by adjusting the urea dosing. A reduced ammonia storage capacity of the SCR catalytic converter or an overdosing is noted by means of an increased ammonia concentration and if necessary also corrected by adjusting the urea dosing.
For optimum operation of the SCR exhaust gas aftertreatment system, a distinct interpretation of the exhaust gas sensor signal is however advantageous, which can occasionally cause difficulties. These difficulties can be met by the procedure according to the invention to interrupt the normal operating mode, if a signal of the exhaust gas sensor is registered which exceeds a predefinable first limit value. The first limit value is preferably given corresponding to a critical nitrogen oxide or ammonia concentration and is preferably in a region between 5 ppm and 50 ppm.
If the first limit value is not exceeded in the normal operating mode, the SCR exhaust gas aftertreatment system operates properly, and there is generally no reason to interrupt the normal operating mode. In the case of an exceeded first limit value, a disturbance is however detected, which is analyzed further according to the invention. That is, according to the invention, if a signal of the exhaust gas sensor is registered as exceeding a predefinable first limit value, the normal operating mode is interrupted, and a urea dosing rate set by the urea dosing device is increased by a predefinable measure compared to the value in the normal operating mode. In the following one changes over into a diagnostic operating mode, if the signal of the exhaust gas sensor increases above a second predefinable limit value which exceeds the first limit value in a predefinable period. The signal profile of the exhaust gas sensor in reaction to the increased urea dosing rate is thus evaluated and it is decided by means of the evaluation, to what extent the initially detected disturbance will need a further diagnosis. The signal of the exhaust gas sensor can be a raw signal or a conditioned output signal of the exhaust gas sensor in the sense of a conventional signal conditioning, which in any case correlates with a nitrogen oxide or ammonia concentration in the exhaust gas.
In the arrangement of the invention, a return is made to the normal operating mode, if the signal of the exhaust gas sensor falls below a predefinable third limit value lying below the first limit value in the predefinable period after the increase of the set urea dosing rate. The falling of the exhaust gas sensor signal seen as correlating with the nitrogen oxide concentration results as a normal reaction of properly working SCR exhaust gas aftertreatment system, in which an improved nitrogen oxide conversion takes place due to an increased offer of urea or ammonia. The SCR exhaust gas aftertreatment system is thus seen to be in proper order in principle, and typically no further error treatments are made, apart from adjustments of the urea dosing which possibly have to take place.
In a further arrangement, a malfunctioning urea dosing device is diagnosed, if the signal of the exhaust gas sensor remains in a predefinable value range including the first limit value in the predefinable period. It is preferably provided to issue an alarm relating to this in this case to cause a service of the urea dosing device. With regard to a dimensioning of the value region which is significant for this, it is especially advantageous, if, in a further arrangement of the invention, this value region is limited by the second limit value to the top and by the third limit value to the bottom. In this manner, a specific differentiation for expected reactions of the exhaust gas sensor is achieved for a urea dosing rate set as increased.
In the case of an exhaust gas sensor signal exceeding the second limit value as a reaction to a urea dosing rate set as increased, a serious disturbance is first assumed, wherein one changes over into the diagnostic mode according to the invention. In the diagnostic operation mode, a catalytic converter characteristic characterizing an updated nitrogen conversion activity is determined in a further arrangement of the invention. The catalytic converter characteristic to be determined is advantageously a nitrogen oxide conversion or a catalytic converter property on hand, as for example the ammonia storage capacity.
If a deviation of the catalytic converter characteristic determined in the diagnostic operating mode from a predefined reference value is determined, an error notification is generated in a further arrangement of the invention, if the deviation is larger than a predefined difference value. An adjustment of the filling level model to the changed catalytic converter characteristic is preferably carried out if the deviation is smaller than the predefined difference value and/or larger than a predefined second lower difference value. The reference value thereby conveniently corresponds to the catalytic converter characteristic of a SCR catalytic converter in an orderly state. The reverse case is also seen to be included thereby, where the reference value corresponds to a limit-value malfunctioning catalytic converter, where the critical difference to an orderly catalytic converter characteristic is already considered.
In a further arrangement of the invention, an internal combustion engine operation with an increased nitrogen oxide raw emission compared to the normal operation is temporarily set in the diagnostic operating mode and the urea dosing rate is reduced in a chronologically synchronous manner or is set to zero. By the increase of the nitrogen oxide supply rate to the catalytic converter with reduced or cut-off urea dosing or ammonia supply and simultaneous monitoring of the exhaust gas sensor signal, the conversion activity of the catalytic converter can be diagnosed. Due to the previous increase of the urea supply rate, it can thereby be assumed that the SCR catalytic converter is supplied optimally with ammonia at the start of the increased nitrogen oxide supply rate and is operated at the best point with regard to this. A specific reference state is thus given for the test of nitrogen oxide activity for the SCR catalytic converter, which enables an especially reliable diagnosis
In a further arrangement, the internal combustion engine operation with an increased nitrogen oxide raw emission is ended, and returned into the normal operating mode, if the signal of the exhaust gas sensor exceeds a predefinable fourth limit value after increasing the nitrogen oxide raw emission. Due to the increased nitrogen oxide offer with a simultaneous reduction or cutoff of the ammonia offer, the SCR catalytic converter has to deal with a nitrogen oxide conversion solely by previously stored ammonia. A using up of this stored ammonia supply thus inevitably leads to an increased nitrogen oxide concentration in the exhaust gas leaving the SCR catalytic converter after some time to be detected by the exhaust gas sensor. For a diagnosis, it is preferred in connection with this procedure according to the invention, if the period until reaching the fourth limit value or the nitrogen oxide amount offered and/or converted in the period are assessed. The nitrogen oxide amount converted within the period is proportional to the amount of the ammonia made available by the SCR catalytic converter in a stored form, which is why the ammonia storage capacity of the SCR catalytic converter can be determined and assessed.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.